1. Field of the Invention
The present invention relates to a cup-shaped grinding wheel for use in machining a machine part made of an aluminum die-cast alloy, cast iron, or the like, and a cup-shaped grinding wheel for use in dressing a polishing pad at the time of CMP processing on a semiconductor wafer.
2. Description of the Related Art
Diamond tools are often used in machining aluminum die-cast alloys, cast iron, etc. Such machining requires high machining efficiency and favorable work surface roughness with fewer scratches.
An example of milling tools fabricated to achieve high machining efficiency is described in Unexamined Japanese Patent Publication No. 2001-79772.
The milling tool described in Unexamined Japanese Patent Publication No. 2001-79772 is a milling tool having an abrasive grain layer, or a tool portion, formed by brazing diamond grains to an end face of a cup-shaped core and an outer periphery thereof. An inclined portion or a curved portion is formed on a part of the end face of the core closer to the outer periphery. The outer periphery of the core and the inclined portion or curved portion of the end face of the core serves as a region for coarse grinding. Abrasive grains are arranged under a condition appropriate for coarse grinding, and a flat part of the end face of the core serves as a region for grinding, abrasive grains are arranged under a condition appropriate for grinding. According to this milling tool, the abrasive grain layer is divided into the region for coarse grinding and the region for grinding, and is provided with abrasive grains under respective appropriate conditions, so that the processing of both coarse grinding and grinding can be performed with the single tool simultaneously for improved machining efficiency.
Meanwhile, dressers for CMP processing often use a dresser having diamond grains firmly fixed to a base. This dresser requires high sharpness and fewer occurrences of wafer scratches resulting from grain cracks and fall-out.
Examples of the dresser for CMP processing having favorable sharpness with less grain cracks and fall-out are described in Unexamined Japanese Patent Publications Nos. 2002-273657 and 2002-126997.
The dresser for CMP processing described in Unexamined Japanese Patent Publication No. 2002-273657 is a dresser for CMP processing in which abrasive grains are firmly fixed to the surface of the base by brazing, with particular crystalline surfaces of these abrasive grains arranged in a certain direction. According to this dresser for CMP processing, the firm fixing of the abrasive grains by brazing provides high sharpness, and the mutual alignment of the directions of the crystalline surfaces of the abrasive grains with each other can suppress the occurrence of grain cracks during dressing.
Moreover, the dresser for CMP processing described in Unexamined Japanese Patent Publication No. 2002-126997 is a dresser for CMP processing in which abrasive grains are firmly fixed to the surface of the base by brazing, and a coating layer comprising glass, as an essential component, having a certain range of coefficients of thermal expansion is applied to the surface of this brazing material layer. According to this dresser for CMP processing, favorable sharpness is provided while erosion of the brazing material layer and the base metal by the abrasive for CMP processing disappears to avoid grain fall-out.
Although the milling tool set forth in Unexamined Japanese Patent Publication No. 2001-79772 and the dresser for CMP processing set forth in Unexamined Japanese Patent Publication No. 2002-273857 described above have favorable grinding capability, they have a problem in terms of grain fall-out. When grain fall-out occurs during grinding, the grain chips move over the surface of the substance to be ground as if dragged around, with the result that there appear big scratches. The timing of the occurrence of scratches resulting from grain fall-out is difficult to predict, and the occurrence of scratches can only be avoided by replacing the grinding wheel earlier to preclude grain fall-out. As a result, the wheel life becomes shorter, which increases the cost of the grinding wheel.
The inventors have made an intensive study of the grain fall-out phenomenon during grinding in the grinding wheel having abrasive grains firmly fixed to the end face of its cup-shaped core by brazing, and confirmed that grain fall-out tends to occur in the outermost peripheral region and innermost peripheral region of the core end face, i.e., in the vicinities of corners. Abrasive grains arranged near the outer peripheral corner of the core end face are apt to fall-out during machining since the brazing material layer formed on the core end face on the outer peripheral side of the abrasive grains has shorter skirts and the brazing material fails to provide sufficient force for holding the abrasive grains. Similarly, the abrasive grains arranged near the inner peripheral corner of the core end face are also apt to fall-out since the brazing material on the inner peripheral side of the core end face provides insufficient force for holding the abrasive grains.
Conventional cup-shaped grinding wheels have not devised a countermeasure against grain fall-out with particular emphasis on the outermost peripheral region and innermost peripheral region of the core end face. The conventional wheels were designed with a principle objective of avoiding grain fall-out over the entire abrasive grain layer, and it has thus been difficult to prevent grain fall-out with a high degree of reliability.
Meanwhile, the dresser for CMP processing set forth in Unexamined Japanese Patent Publication No. 2002-126997 is an effective means in terms of the prevention of grain fall-out, whereas there is the problem that the application of the additional coating layer to the surface of the brazing material layer decreases the heights of protrusion of the abrasive grains accordingly with a drop in sharpness, and shrinks chip pockets between abrasive grains with a drop in the capability of ejecting chips.
The present invention has been achieved in order to solve such problems, and it is thus an object thereof to provide a grinding wheel which can preclude the occurrence of scratches resulting from grain fall-out to obtain a favorable work surface.